첫째, 사건을 변환
친구가 최근에 다음과 같이 이중 문자열로 변환 정밀도 손실의 사건을 논의하기 :
친구 쑤 Chenliang에서이 사건
루트 @의 mysqldb 22시 12분 : [xucl]> 쇼를 만들 테이블 T1의 \의 G *************************** 1. 행 *** ************************ 테이블 : t1Create 테이블 : CREATE TABLE`t1` (`id`의 VARCHAR를 (255) DEFAULT NULL) ENGINE = 이노 DEFAULT 문자셋 = 세트 utf81 행 (0.00 초)의 루트 @ mysqldb 22시 19분 [xucl]> T1로부터 선택 *] + -------------------- + | ID | + -------------------- + | 204027026112927605 || 204027026112927603 || 2040270261129276 || 2040270261129275 || 100 || 101 | + --------- ----------- 세트 + 6 행 (0.00 초) 이상 현상 : 루트 @의 mysqldb 22시 19분 [xucl]> T1로부터 선택한 * 여기서 식 204,027,026,112,927,603; + ----- --------------- + | ID | + -------------------- + | 204027026112927605 || 204027026112927603 | + - ------------------ 지옥, 그것은 204027026112927605, 검사가 204027026112927603입니다 분명 어떤 설정 (0.00 초)에서 + 2 행은 왜 나온
공식 문서가 매우 명확하게 작성 변환은, 여기에 양쪽는 비교 double 형으로 변환됩니다.
둘째, 간단한 테스트
사실, 우리는이 조건을 테스트 할 수있는 필드의 두 유형의 문자열을 삽입 할 수 있습니다 :
MySQL이> ( '1111111111111111111111111111111111')를 test11 값 삽입> -ADatabase changedmysql와 빠른 시동을 얻기 위해이 기능을 끌 수있는 테이블과 열 namesYou의 완료 testmtsReading 테이블 정보를 사용하여, 쿼리 OK를, 1 행의 영향 (37 분 53.07 초) MySQL의> 쇼 테이블을 만들 test11 + -------- + --------------------------------- -------------------------------------------------- ----- + | 표 | 표 만들기 | + -------- + ------------------------------------- -------------------------------------------------- - + | test11 | 표`test11`를 CREATE (`n` 더블 DEFAULT NULL) ENGINE = InnoDB의의 디폴트 캐릭터 세트 = UTF8 | + -------- + ------------------- -------------------------------------------------- ------------------- 세트 + 1 행 (0.00 초)의 MySQL> test11 SELECT * FROM + ----------------------- + | N | + ----------------------- + | 9.007199254740991e15 || 9.007199254740992e15 || 9.007199254740991e17 || 1.1111111111111112e31 || 1.1111111111111112e31 || 1.1111111111111111e33 || 1.1111111111111111e33 || 1.1111111111111111e33 || 1.1111111111111111e33 || 1.1111111111111111e33 || 1.1111111111111111e33 | + ----------------------- + 세트에 11 행 (0.00 초)
우리는 사실의 정확성이 손실되는 것을 볼 수 있습니다. 아마 17 사이의 16 자 문자 및 가능성은 정밀 문자열 손실을 무시합니다. 정주 불임 병원 : HTTP : //jbk.39.net/yiyuanzaixian/zztjyy/
셋째, 전달 함수
라인의 수천이 변환 기능 수백, 너무 열심히, 조심스럽게 보이지 않았다. 이 프레임을 스택 기록 : https://www.jianshu.com/p/fc56f6221728
#0 my_strtod_int (s00=0x7ffedc0065f0 '1' <repeats 34 times>, se=0x7fffec5eb678, error=0x7fffec5eb680, buf=0x7fffec5ea770 "\320\001", buf_size=3680) at /mysqldata/percona-server-locks-detail-5.7.22/strings/dtoa.c:1356#1 0x0000000001ee07c1 in my_strtod (str=0x7ffedc0065f0 '1' <repeats 34 times>, end=0x7fffec5eb678, error=0x7fffec5eb680) at /mysqldata/percona-server-locks-detail-5.7.22/strings/dtoa.c:472#2 0x0000000001ec4eda in my_strntod_8bit (cs=0x2e379e0, str=0x7ffedc0065f0 '1' <repeats 34 times>, length=34, end=0x7fffec5eb678, err=0x7fffec5eb680) at /mysqldata/percona-server-locks-detail-5.7.22/strings/ctype-simple.c:741#3 0x0000000000f4dbae in Field_double::store (this=0x7ffedc9a1390, from=0x7ffedc0065f0 '1' <repeats 34 times>, len=34, cs=0x2e379e0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/field.cc:4825#4 0x0000000000f9bbc7 in Item::save_str_value_in_field (this=0x7ffedc006618, field=0x7ffedc9a1390, result=0x7ffedc006628) at /mysqldata/percona-server-locks-detail-5.7.22/sql/item.cc:561#5 0x0000000000fae204 in Item_string::save_in_field_inner (this=0x7ffedc006618, field=0x7ffedc9a1390, no_conversions=false) at /mysqldata/percona-server-locks-detail-5.7.22/sql/item.cc:6895#6 0x0000000000fad7de in Item::save_in_field (this=0x7ffedc006618, field=0x7ffedc9a1390, no_conversions=false) at /mysqldata/percona-server-locks-detail-5.7.22/sql/item.cc:6752#7 0x0000000001549101 in fill_record (thd=0x7ffedc000b90, table=0x7ffedc9a8650, ptr=0x7ffedc9a1388, values=..., bitmap=0x0, insert_into_fields_bitmap=0x0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_base.cc:9702#8 0x00000000015493e9 in fill_record_n_invoke_before_triggers (thd=0x7ffedc000b90, ptr=0x7ffedc9a1380, values=..., table=0x7ffedc9a8650, event=TRG_EVENT_INSERT, num_fields=1) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_base.cc:9784#9 0x00000000017fd477 in Sql_cmd_insert::mysql_insert (this=0x7ffedc006d80, thd=0x7ffedc000b90, table_list=0x7ffedc0067c8) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_insert.cc:751#10 0x00000000018040ed in Sql_cmd_insert::execute (this=0x7ffedc006d80, thd=0x7ffedc000b90) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_insert.cc:3121#11 0x00000000015cc753 in mysql_execute_command (thd=0x7ffedc000b90, first_level=true) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:3746#12 0x00000000015d2fde in mysql_parse (thd=0x7ffedc000b90, parser_state=0x7fffec5ed600) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:5901#13 0x00000000015c6b72 in dispatch_command (thd=0x7ffedc000b90, com_data=0x7fffec5edd70, command=COM_QUERY) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:1490#14 0x00000000015c58ff in do_command (thd=0x7ffedc000b90) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:1021#15 0x000000000170e578 in handle_connection (arg=0x6ab5a00) at /mysqldata/percona-server-locks-detail-5.7.22/sql/conn_handler/connection_handler_per_thread.cc:312#16 0x0000000001945538 in pfs_spawn_thread (arg=0x697a460) at /mysqldata/percona-server-locks-detail-5.7.22/storage/perfschema/pfs.cc:2190#17 0x00007ffff7bcfaa1 in start_thread () from /lib64/libpthread.so.0#18 0x00007ffff6b37c4d in clone () from /lib64/libc.so.6
开头案例转换栈帧
#0 my_strtod (str=0x7ffee097a803 "204027026112927605", end=0x7fff009ef778, error=0x7fff009ef784) at /mysqldata/percona-server-locks-detail-5.7.22/strings/dtoa.c:468#1 0x0000000001ec4eda in my_strntod_8bit (cs=0x2e377a0, str=0x7ffee097a803 "204027026112927605", length=18, end=0x7fff009ef778, err=0x7fff009ef784) at /mysqldata/percona-server-locks-detail-5.7.22/strings/ctype-simple.c:741#2 0x0000000000f5639f in Field_varstring::val_real (this=0x7ffee097ae60) at /mysqldata/percona-server-locks-detail-5.7.22/sql/field.cc:7663#3 0x0000000000fa2b9d in Item_field::val_real (this=0x7ffee0007180) at /mysqldata/percona-server-locks-detail-5.7.22/sql/item.cc:2960#4 0x0000000000fc8d3d in Arg_comparator::compare_real (this=0x7ffee0007378) at /mysqldata/percona-server-locks-detail-5.7.22/sql/item_cmpfunc.cc:1745#5 0x0000000000fde0fc in Arg_comparator::compare (this=0x7ffee0007378) at /mysqldata/percona-server-locks-detail-5.7.22/sql/item_cmpfunc.h:92#6 0x0000000000fcafb9 in Item_func_eq::val_int (this=0x7ffee00072a0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/item_cmpfunc.cc:2507#7 0x0000000001581a11 in evaluate_join_record (join=0x7ffee00077b8, qep_tab=0x7ffee0a77520) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_executor.cc:1492#8 0x0000000001581372 in sub_select (join=0x7ffee00077b8, qep_tab=0x7ffee0a77520, end_of_records=false) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_executor.cc:1297#9 0x0000000001580be6 in do_select (join=0x7ffee00077b8) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_executor.cc:950#10 0x000000000157eaa2 in JOIN::exec (this=0x7ffee00077b8) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_executor.cc:199#11 0x0000000001620327 in handle_query (thd=0x7ffee0000c00, lex=0x7ffee0003230, result=0x7ffee00074e8, added_options=0, removed_options=0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_select.cc:185#12 0x00000000015d1f77 in execute_sqlcom_select (thd=0x7ffee0000c00, all_tables=0x7ffee0006be0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:5445#13 0x00000000015ca380 in mysql_execute_command (thd=0x7ffee0000c00, first_level=true) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:2939#14 0x00000000015d2fde in mysql_parse (thd=0x7ffee0000c00, parser_state=0x7fff009f1600) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:5901#15 0x00000000015c6b72 in dispatch_command (thd=0x7ffee0000c00, com_data=0x7fff009f1d70, command=COM_QUERY) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:1490#16 0x00000000015c58ff in do_command (thd=0x7ffee0000c00) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:1021#17 0x000000000170e578 in handle_connection (arg=0x3605a30) at /mysqldata/percona-server-locks-detail-5.7.22/sql/conn_handler/connection_handler_per_thread.cc:312#18 0x0000000001945538 in pfs_spawn_thread (arg=0x36e71c0) at /mysqldata/percona-server-locks-detail-5.7.22/storage/perfschema/pfs.cc:2190#19 0x00007ffff7bcfaa1 in start_thread () from /lib64/libpthread.so.0#20 0x00007ffff6b37c4d in clone () from /lib64/libc.so.6
四、官方文档关于转换的说明
The following rules describe how conversion occurs for comparison operations:
• If one or both arguments are NULL, the result of the comparison is NULL, except for the NULL-safe
<=> equality comparison operator. For NULL <=> NULL, the result is true. No conversion is needed.
• If both arguments in a comparison operation are strings, they are compared as strings.
• If both arguments are integers, they are compared as integers.
• Hexadecimal values are treated as binary strings if not compared to a number.
• If one of the arguments is a TIMESTAMP or DATETIME column and the other argument is a
constant, the constant is converted to a timestamp before the comparison is performed. This is
done to be more ODBC-friendly. This is not done for the arguments to IN(). To be safe, always
use complete datetime, date, or time strings when doing comparisons. For example, to achieve best
results when using BETWEEN with date or time values, use CAST() to explicitly convert the values to
the desired data type.
A single-row subquery from a table or tables is not considered a constant. For example, if a subquery
returns an integer to be compared to a DATETIME value, the comparison is done as two integers.
The integer is not converted to a temporal value. To compare the operands as DATETIME values,
use CAST() to explicitly convert the subquery value to DATETIME.
• If one of the arguments is a decimal value, comparison depends on the other argument. The
arguments are compared as decimal values if the other argument is a decimal or integer value, or as
floating-point values if the other argument is a floating-point value.
• In all other cases, the arguments are compared as floating-point (real) numbers.
五、为什么会丢失精度
一般来讲double占用8直接也就是64位其中：
1位 符号位
11位 指数位
52位 尾数位
我们可以看到尾数位才是精度的保证，它的精度实际上没有long int类型高。因此当查过保存的上限将会丢失精度。